Downloading of graphics, e.g., via the Internet/WWW or any other network, is a slow process if, for example, the bandwidth is limited or the server is overloaded.
For example, a 100 KByte GIF (Graphics Interchange Format) file requires 30 seconds or more to download with 28.8K modem. GIF image data is always stored by scanline and by pixel. Also, GIF can store only images with a maximum pixel depth of 8 bits, for a maximum of 256 colors. Using a GIF with a scheme of interlacing scanlines permits viewers to see enough of a clickable image map to navigate it in about half the time, i.e., 15 seconds in the example above.
PNG (Portable Network Graphics) is another lossless image format, capable of processing more than 256 colors and having somewhat better compression performance than GIF. Navigation is possible in a time period even shorter than for the interlaced GIF, owing to the two-dimensional interlacing method used in the PNG format.
Progressive JPEG is an excellent lossy compression method for, e.g., natural scenes (typically a pixel depth of 6 to 24 bits). The JPEG images are typically of smaller size than comparable GIF or PNG images. However, JPEG is not really suited to process animations, ray tracing, line art, black-and-white documents, and typical vector graphics.